The present invention relates generally to head drive mechanisms, and more particularly to a mechanism for fine tracking the position of a head. The present invention is suitable, for example, for a hard disc drive (“HDD”) which has a plurality of magnetic discs.
Available electronic information content is explosively increasing with the recent rapid technology development, as in the Internet. Accordingly, smaller and larger-capacity magnetic disc drives, typified by HDDs, have been increasingly demanded to store such a large amount of information. The increased number of data tracks per unit length (or TPI: Track per Inch), that is, a narrow track width is essential to realize a smaller and larger-capacity HDD. In addition, the improved head positioning accuracy is also required for reading data onto and reproducing data from a narrow track.
A so-called micro-actuator has been proposed as an effective means for improving positioning accuracy, which includes, in addition to a conventional head actuator mechanism, means for fine positioning (or minutely moving) a head device (or a slider equipped with the head device or a suspension that supports the slider). The instant inventor proposed an actuator that uses shear of a piezoelectric element as one example of such micro-actuators in Japanese Laid-Open Patent Application No. 2001-43641. This actuator has a layered structure including a suspension that supports a head device, a hinge plate that deforms as the piezoelectric element shears, two pairs of piezoelectric elements and electrodes, an actuator base, and an arm that supports the actuator base. However, the conventional shear type piezoelectric actuator has the following disadvantages:
First, due to the layered structure, a tolerance of the actuator depends upon a dimension tolerance of each member, and necessarily varies widely. Therefore, the suspension's attachment dimension accuracy is poor, resulting in difficult mounting of plural discs and/or fluctuating spring pressure with which the slider contacts a disc. In addition, where each of a pair of piezoelectric elements has a different thickness, the suspension is likely to be fixed while inclined with respect to an arm attachment surface. The fluctuating spring pressure and inclined suspension can result in a head crash (i.e., damage to a disc) and lowered positioning accuracy.
Second, the conventional shear type piezoelectric actuator barely enlarges the minutely deformed amount (or stroke). For example, it is known that an actuator that uses a longitudinal effect of the piezoelectric element may enlarge the stroke, but it is difficult to layer the shear type element for manufacturing reasons and thus hard to provide a sufficiently large stroke. Access to a track beyond fine positioning requires driving by a voice coil motor, resulting in a low head positioning speed. A large stroke for fine tracking purposes shortens the time period necessary for settling (which is a setting operation to a target position in a head positioning operation), and effectively provides faster settling. Disadvantageously, a small fine tracking stroke is likely to be saturated under a strong disturbance, such as external vibrations, and cannot provide desired compensation.